The present invention relates to xerographic printing apparatus and methods, and, more specifically, relates to systems and methods that can extend the useful life of a charge receptor, such as a photoreceptor.
Conventional electrostatographic printing or reproduction apparatus, such as xerographic devices, include a print engine that utilizes a charge receptor, such as a photoreceptor (PR), to receive an electrostatic, latent image which conforms to an image desired to be produced (for example, copied or printed). Toner is then attracted to the charge receptor in amounts proportional to the localized charge of the electrostatic latent image. Thereafter, the toner is transferred to another belt or drum, or to a transfer medium such as a sheet of paper or other media.
To create an electrostatic image on the charge receptor, many xerographic engines, particularly color xerographic engines, make use of contact and/or close proximity charging devices, including biased charge rollers (BCRs). Such charging devices operate to create a sufficient voltage between the charge receptor and the BCR so that the threshold breakdown voltage, VTH, of the air between the charging device and the charge receptor is met or exceeded. The threshold voltage varies with the particular geometry of the charging device and charge receptor. When the threshold voltage is exceeded, a corona plasma is generated in the nip region, which is the region between the charge receptor and the charging device. For contact charging devices, the nip region is the region just before the charging device and charge receptor make contact and immediately after the region the charging device and receptor make contact. The charge receptor surface is charged from the corona plasma.
Although the charging device itself may contact the charge receptor, contact is not a necessary condition for the corona to contact or reside in close proximity to the charge receptor. Further, the intense corona generation near the receptor surface can contribute to high rates of charge receptor wear.
A DC voltage may be used to drive the charging device. However, such a driving voltage does not produce a sufficiently uniform charge on the charge receptor for many applications. DC only charging devices are typically used for low end black and white machines or very short life xerographic units because of the lack of uniformity in the charge receptor charge. Thus, the conventional waveform for driving contact and/or close proximity type charging devices is an AC voltage waveform superimposed on a DC bias voltage (AC+DC charging). Conventionally, the AC voltage waveform is a sine wave. Such driving waveforms produce a charge receptor charge having superior uniformity. Additionally, such a waveform guards against contamination and provides some erase functionality. However, one significant drawback to AC+DC charging is the amount of positive corona plasma that is generated near the nip formed between the charging device and the charge receptor surface.
FIG. 1 shows a graph of a typical response of the charge receptor potential as a function of the AC peak-to-peak voltage (actuator) input to a charging device. The location of the charging device saturation point in this curve (the point at which further increases in the actuator do not significantly affect the output photoconductor charge voltage) is typically referred to as the “knee” of the charge curve, or the inflection point. In the example of FIG. 1, the knee occurs at approximately 1400 volts (V) on the AC peak-to-peak voltage axis resulting in a photoreceptor potential of approximately 750 volts. Typically, non-uniform print quality is obtained for AC charging devices when the AC peak-to-peak actuator is operated below this knee value. In addition, under certain conditions, some print quality defects may occur for actuator values close to, but above the knee of the charge curve.
One defect that can occur is the production of light and dark spots (sometimes referred to as salt-and-pepper noise) which occurs between the charging knee and a Vp-p (peak-to-peak voltage) value known as the background disappearing point (“BDP”). The spots can be black (on white backgrounds) or white (on black backgrounds). The light and dark spots that appear as a result of the BDP defect are typically referred to as BDP spots. To prevent BDP spots from occurring, it is conventional to maintain the AC charging actuator at a VP-P voltage value sufficiently above the BDP. Thus, in most xerographic engines that make use of contact and/or close proximity AC charging devices, the charging actuator is operated at a value sufficiently far above the knee of the curve to ensure acceptable output print quality despite variations in the process. Generally, the BDP is 100 to 200 volts above the knee, and thus, conventionally, AC+DC charge devices are operated 200 to 400 volts above the knee. Thus, a conventional AC+DC charging waveform is a 1500 to 2500 volts peak-to-peak sinusoidal AC waveform biased by a DC voltage bias. The DC bias is chosen depending on the other xerographic subsystems, speed of the charge receptor, and type of toner material being used, but may be between −500 and −800 volts.
Because photoreceptors are typically somewhat expensive to replace, the life of these devices can have a significant impact on the overall run cost of the print engine. In fact, this can be one of the largest contributors to the parts costs for many tandem color xerographic machines.
A problem with conventional contact and/or close proximity AC charge devices operated at or above the BDP is that the rate of wear of the photoreceptor is accelerated as a result of positive ion deposition onto the photoreceptor surface by the charging device. These positive ions are believed to interact with the surface of the photoreceptor, degrading the binder molecules such as polycarbonate binder resin molecules, thereby making the photoreceptor more susceptible to abrasion and wear. It is believed that the weakening of the binder molecules results from an electrochemical interaction between the positive ions and the binder molecules, or damage due to the kinetic energy of the positive ions impinging the binder molecules. Thus, as the photoreceptor is cleaned of residual toner after image transfer by a cleaning blade, wear is accelerated. The greater the number of positive ions deposited onto the surface of the photoreceptor during charging, the more quickly the photoreceptor surface material will wear.
In addition, the larger the amount by which the charge knee voltage is exceeded, the larger the amounts of both positive and negative ions that will be produced during each cycle of the charging waveform. That is, the magnitude of the AC charging voltage applied to the charging device can significantly affect the amount of positive charge deposition that occurs on the photoreceptor surface. For a given DC offset voltage, larger peak-to-peak amplitudes for the applied AC voltage above the charging knee will typically lead to larger amounts of positive charge deposited onto the PR surface for each charging cycle. The larger the amount of positive charge deposited onto the photoreceptor surface by the charging device, the faster the PR surface will wear. Thus, it is highly desirable to minimize the distance of the charging actuator above the knee of the charge curve at all times.
In an effort to limit the amount of positive charge deposited onto the surface of the photoreceptor while maintaining acceptable output print quality, some prior methods have attempted to reduce the peak-to-peak magnitude of the AC voltage waveform in an attempt to reduce the production of positive ions. However, if reduced too much, BDP spots appear. Others attempted modulation of the AC waveform in different ways. However, many of these techniques result in difficulty with process control techniques and poor halftone uniformity in the resulting images.
Other efforts to address the need for longer life photoreceptor devices in systems with contact and/or close proximity AC charging have focused on materials related solutions. These types of approaches can include such things as improved overcoats on the photoreceptors to make them more durable. Unfortunately, these types of solutions are somewhat difficult to develop and can, in fact, cause other problems in the system. For example, creating a harder photoreceptor surface in a xerographic system with a blade cleaning device shifts the wear problems from the receptor surface to the cleaner blade edge, which can lead to reduced cleaning blade life, which might not allow a significant gain in system run cost to be realized through such a materials based solution.
Still other methods have looked at using non-contact charging devices or other subsystem changes to reduce the abrasion of the photoreceptor surface. For example, a non-contact charging device, such as a scorotron, applies high voltage to a wire or pin coronode located a distance, such as about 5 mm or more, from the photoreceptor surface. The ion generating corona discharge is localized around the coronode is such devices, not touching, but in relatively close proximity to the photoreceptor. This method of receptor charging results in generation of dysfunctional bi-products in the form of ozone and NOx, which are both harmful to the receptor and the environment in general.
Additional prior methods, such as, for example, that disclosed in U.S. Pat. No. 7,024,125, have suggested mechanisms for adjusting the charging actuator in an active fashion. However, these prior methods are limited in the information that they use to adjust the charging actuator. Such methods are typically limited to measurement of a current as a mechanism for measuring the charge level of the photoreceptor. Unfortunately, for some devices, such as biased-transfer rolls, the measurement of a current using a constant voltage mode of operation can be quite noisy. For example, if the impedance of any component changes, this can have a detrimental effect on the current measurement. In addition, prior methods typically do not make use of image quality information in their adjustment of the charging actuators. Instead, these prior systems are limited to measurements only of the underlying process parameters, namely the location of the charging knee, or threshold voltage, through measurement of a downstream current flow. While this method does give an idea of the voltage required to yield good print quality, the BDP location can't be determined reliably without feedback in the form of image quality.
Thus, there remains a need for a xerographic system with a charging device that will optimize photoreceptor life in a robust fashion while ensuring that charging related print quality defects do not occur.